1. Field of the Invention
The invention relates generally to a variable valve timing system and a method for controlling the same, and, more specifically, to a variable valve timing system that is provided with a mechanism which changes opening/closing timing of a valve by an amount of change corresponding to an operation amount of an actuator, and a method for controlling the same.
2. Description of the Related Art
A variable valve timing (VVT) system that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing system changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
For example, Japanese Patent Application Publication No. JP-A-2005-120874 (JP-A-2005-120874) describes a valve timing adjustment device that adjusts the valve timing of a valve provided in an engine using a rotary torque produced by an electric motor. The valve timing adjustment device sets a target amount of change in the rotational speed of the electric motor based on the deviation of the actual phase, which is determined based on the rotational speed of a crankshaft and the rotational speed of a camshaft, from the target phase set based on the operating state of the engine. The target amount of change corresponds to the rate of phase change, and the electricity passing through the electric motor is controlled by a drive circuit that receives a control signal indicating the target amount of change in the rotational speed of the electric motor.
The valve timing of a valve provided in an engine exerts a great influence on the combustion stability, the fuel efficiency, the power output from the engine, exhaust emission, etc. Namely, the target phase of the valve timing varies depending on which of the above-mentioned elements is given a priority. For example, when the engine is idling, the target phase at which a priority is given to the combustion stability is set.
Generally, the target phase is set in advance based on the operating state of the engine such that the above-mentioned elements are collectively realized in a balanced manner. More specifically, while the engine is operating, the target phase of the valve timing is successively set in accordance with a change in the operating state of the engine with reference to, for example, a map that stores the correlation between the engine operating state and the target phase in advance.
Accordingly, during the valve timing control, a valve timing change that reduces the combustion stability is sometimes made. Therefore, it is important to take the correlation between the direction in which the valve timing is changed and the combustion stability into account in order to execute the valve timing control to improve the total engine performance as described above. When the phase at which the combustion stability is high is present in the middle of the control range in which the phase of the valve timing is changed, the rate of phase change is changed depending on whether the phase is advanced or delayed. In addition to this, the control should be executed with the correlation between the direction in which the valve timing is changed and the combustion stability taken into account.